Temporary hard water is formed when rain water containing dissolved carbon(IV) oxide flows over deposits of

Analysis of the Question

The question asks for the specific mineral deposit that, when reacted with rainwater containing dissolved carbon(IV) oxide ($CO_2$), results in the formation of temporary hard water.

Concept Explanation

Temporary hardness in water is caused by the presence of dissolved calcium hydrogen trioxocarbonate(IV) or magnesium hydrogen trioxocarbonate(IV).

When rainwater falls, it absorbs $CO_2$ from the atmosphere to form a weak acid called trioxocarbonate(IV) acid ($H_2CO_3$):

When this acidic water flows over limestone deposits (which is calcium trioxocarbonate(IV), $CaCO_3$), the acid reacts with the insoluble limestone to form soluble calcium hydrogen trioxocarbonate(IV):

The resulting $Ca(HCO_3)_2$ makes the water "temporarily hard" because the hardness can be removed by simply boiling the water, which converts the soluble hydrogen trioxocarbonate(IV) back into insoluble $CaCO_3$.

Step-by-Step Solution

1. Identify the cause: Temporary hardness is caused by $Ca(HCO_3)_2$ or $Mg(HCO_3)_2$.
2. Identify the source: These are formed when rainwater (containing $CO_2$) reacts with insoluble carbonates like $CaCO_3$.
3. Evaluate the options: • A. $CaCO_3$ (Calcium trioxocarbonate(IV)): This is limestone. It reacts with $CO_2$ and water to form soluble $Ca(HCO_3)_2$. This is the correct source. • B. $Na_2CO_3$: Sodium salts are generally soluble and do not cause hardness in water. • C. $Na_2SO_4$: Sodium salts do not cause hardness. • D. $CaSO_4$: Calcium tetraoxosulphate(VI) causes permanent hardness, not temporary hardness, because it does not decompose upon boiling.

Conclusion

The reaction between rainwater containing $CO_2$ and limestone ($CaCO_3$) is the classic process for creating temporary hard water in nature.